Pixel driving device for a liquid crystal display

ABSTRACT

A pixel driving device for a liquid crystal display. The pixel driving device includes a thin film transistor (TFT), a pixel capacitor, a storage capacitor, a first common line, and a second common line. The TFT includes a gate electrode coupled to a scan line and a source coupled to the data line. The pixel capacitor is coupled to the TFT drain electrode and the first common line, while the storage capacitor is coupled to the TFT drain electrode and the second common line. The first common line and the second common line possess different common voltages. When a pixel voltage is applied to the pixel, the pixel capacitor and the storage capacitor possess different capacitor voltage values by being coupled to different common lines, respectively.

[0001] This application claims the benefit of Taiwan application Serial No. 091102288, filed on Feb. 7, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a pixel driving device, and more particularly to a pixel driving device for a liquid crystal display.

[0004] 2. Description of the Related Art

[0005] Display technology has seen great advances. Conventional cathode ray tubes (CRTs) have been gradually superseded by liquid crystal display (LCD) in the high-end display market. CRTs have some major drawbacks such as large size and high radiation emissions while LCD monitors have advantages of no radiation emissions, low power consumption, and lightweight.

[0006]FIG. 1 is a schematic diagram showing a pixel driving device for a pixel in a conventional thin film transistor liquid crystal display (TFT-LCD) panel. The LCD panel includes a plurality of pixels arranged as a matrix. Each pixel has a pixel driving device for driving liquid crystal molecules of the pixel. The pixel driving device includes a thin film transistor (TFT) having a gate electrode coupled to a scan line S_(N) and a source electrode coupled to a data line D_(M). The pixel driving device further includes a pixel capacitor C_(LC) and a storage capacitor C_(ST) wherein the storage capacitor C_(ST) stores charges to hold a voltage across the pixel capacitor C_(LC), thus keeping the gray scale of the pixel stable. A drain electrode of the TFT is coupled to the pixel capacitor C_(LC) and the storage capacitor C_(ST). The storage capacitor C_(ST) and the pixel capacitor C_(LC) are connected in parallel to a common line L_(COM). The connection for the storage capacitor C_(ST) is called a conventional “C_(ST) on common” mode.

[0007] When the LCD displays frames, a drive circuit sequentially enables each scan line and turns on the TFTs of each row of pixels on the panel. Meanwhile, the drive circuit sequentially applies pixel voltages Vp from the data line corresponding to each of the pixels. The pixel voltage Vp is applied to the pixel capacitor C_(LC) and the storage capacitor C_(ST). Meanwhile, the common line also provides a common voltage. The capacitor voltages of the pixel capacitor C_(LC) and the storage capacitor C_(ST) are determined according to the voltage difference of the common voltage and the pixel voltage Vp. The pixel capacitor voltage difference is utilized to drive the liquid crystal molecules of the pixel giving the pixel a desired gray scale value while the storage capacitor voltage difference is utilized to hold the desired gray scale stable. Since the storage capacitor C_(ST) and the pixel capacitor C_(LC) are connected in parallel to the common line L_(COM), the values of the capacitor voltages of the pixel capacitor C_(LC) and the storage capacitor C_(ST) are the same.

[0008]FIGS. 2A to 2B illustrate the arrangement of the liquid crystal molecules in a twisted nematic (TN) mode liquid crystal panel with and without the pixel voltage Vp applied, respectively. In FIGS. 2A and 2B, the arrows show the indicating directions of a front-plate alignment film 204 and a rear-plate alignment film 202 in the TN mode liquid crystal panel. In particular, the indicating directions of the front-plate alignment film 204 and the rear-plate alignment film 202 are perpendicular to each other. The directions of long axes of the liquid crystal molecules 200 close to the alignment films 202 and 204 are substantially parallel to the indicating directions of the alignment films 202 and 204, respectively. When no pixel voltage Vp is applied, the liquid crystal molecules 200 gradually twist until the uppermost layer close to the front-plate alignment film 204 is at a 90-degree angle to the rear-plate alignment film 202, as shown in FIG. 2A. Under these conditions, the liquid crystal molecules 200 possess high light transmission rates, and the pixel's brightness reaches a maximum. FIG. 2B shows that when the proper pixel voltage Vp is applied, the liquid crystal molecules 200 are rotated to be in parallel with the direction of the electric field. In this case, the liquid crystal molecules 200 possess low light transmission rate, and the brightness of the pixel is reduced.

[0009] During the manufacture of the panel, the gate electrode of the TFT and the lower electrode of the storage capacitor C_(ST) for a pixel are formed in one manufacturing step. In addition, the drain and source electrodes of the TFT, and the upper electrode of the storage capacitor C_(ST) for the pixel are all formed in another manufacturing step. For the sake of description, the gate electrode of the TFT and the lower electrode of the storage capacitor C_(ST) are referred to as a first metal layer M1, while the drain and source electrodes of the TFT and the upper electrode of the storage capacitor C_(ST) are referred to as a second metal layer M2. A silicon nitride (SiN_(x)) layer is provided between the lower electrode and the upper electrode of the storage capacitor C_(ST) to serve as a dielectric material between the two plates of the storage capacitor C_(ST).

[0010] Due to the possibility for error when manufacturing the panels, the silicon nitride layer between the lower electrode and the upper electrode of the storage capacitor C_(ST) may be doped with impurities or other substances, or voids may be formed in the silicon nitride layer. If this occurs, the first metal layer and the second metal layer are short-circuited. If the two metal layers short-circuit, the electrical potentials of the lower and upper electrodes of the storage capacitor C_(ST) for the pixel are equal regardless of the magnitude of pixel voltage Vp applied to the pixel. The voltage difference between the lower and upper electrodes of the pixel of the liquid crystal panel would be zero. The pixel in this case is faulty. In a TN mode liquid crystal panel, when the above-mentioned problem occurs in the storage capacitor of a pixel, the faulty pixel always displays its brightness regardless of the applied pixel voltage Vp, and causes a bright spot, especially, for a normally white TN mode liquid crystal panel. When the liquid crystal panel has a bright spot, the display quality of the liquid crystal panel is seriously degraded and customers are not willing to buy these products.

SUMMARY OF THE INVENTION

[0011] In view of the above-mentioned problems, it is therefore an object of the invention to provide a pixel driving device for a liquid crystal display (LCD) with no bright spot. When a pixel of the LCD becomes faulty due to a short-circuit between a first metal layer and a second metal layer of the pixel storage capacitor, a bright spot is prevented from appearing on the liquid crystal panel. The influence of panel manufacturing errors upon the display quality of the liquid crystal panel can thus be reduced.

[0012] The invention discloses a pixel driving device for a pixel of a liquid crystal display. Each pixel is coupled to a scan line and a data line. The pixel driving device includes a thin film transistor (TFT), a pixel capacitor, a storage capacitor, a first common line, and a second common line. The TFT includes a gate electrode coupled to the scan line, a source electrode coupled to the data line, and a drain electrode. The pixel capacitor is coupled between the drain electrode of the TFT and the first common line, while the storage capacitor is coupled between the drain electrode of the TFT and the second common line. The first common line has a first common voltage while the second common line has a second common voltage. When a pixel voltage is applied to the pixel, the pixel capacitor and the storage capacitor possess different capacitor voltage values by being coupled to respective common lines.

[0013] The invention further discloses a driving device for a liquid crystal display. The driving device includes a scan line, a data line, and a TFT. The TFT includes a gate electrode coupled to the scan line, a source electrode coupled to the data line, and a drain electrode. The driving device may further include a first power source, a second power source, a pixel capacitor coupled between the drain electrode and the first power source, and a storage capacitor coupled between the drain electrode and the second power source. The first power source provides a first voltage, and the second power source provides a second voltage level which is different from the first voltage level.

[0014] Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiment. The following description is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic diagram showing a pixel driving device for a pixel in a conventional TFT-LCD.

[0016]FIGS. 2A to 2B are schematic diagrams showing the arrangement of the liquid crystal molecules in a twisted nematic (TN) mode liquid crystal panel with and without the pixel voltage Vp applied, respectively.

[0017]FIG. 3 is a schematic diagram showing a pixel driving device for a pixel in a TFT-LCD of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The storage capacitor and pixel capacitor are connected to two different common lines with two different common voltages giving them different capacitor voltage values. When the first metal layer and the second metal layer of a pixel are short-circuited unintentionally, a capacitor voltage can be maintained across the pixel capacitor by disconnecting the source/drain electrodes from the storage capacitor, for example, with a laser. The capacitor voltage of the pixel capacitor is large enough to arrange the liquid crystal molecules of the pixel in a direction parallel to the electric field, which are arranged between the front and rear plates of the liquid crystal panel, such as a normally white (NW) mode TN LCD. Consequently, the brightness of such faulty pixel can be decreased, preventing a bright spot on the liquid crystal panel.

[0019] Referring to FIGS. 3, a pixel driving device for a pixel in a TFT-LCD is illustrated according to a preferred embodiment of the invention. In this embodiment, a pixel capacitor C_(LC) and a storage capacitor C_(ST) are respectively coupled to a first common line Lcom1 and a second common line Lcom2. The first common line Lcom1 and the second common line Lcom2 are coupled to a first power source and a second power source which can be positioned outside the liquid crystal panel, respectively. The first power source provides a first common voltage Vcom1, and the second power source provides a second common voltage Vcom2, wherein the first common voltage Vcom1 and the second common voltage Vcom2 are of different voltage levels. In addition to the second common line Lcom2, the second power source may also be connected to the gate electrode of the TFT for the pixel. In this configuration, the first common line Lcom1 provides the first common voltage Vcom1 while the second common line Lcom2 provides the second common voltage Vcom2. When the pixel voltage Vp is applied to the pixel, the capacitor voltage value of the pixel capacitor C_(LC) is determined by the pixel voltage Vp and the first common voltage Vcom1. The capacitor voltage value of storage capacitor C_(ST) is determined by the pixel voltage Vp and the second common voltage Vcom2. By being coupled to the different common lines with different common voltages, the pixel capacitor C_(LC) and the storage capacitor C_(ST) may have different capacitor voltage values.

[0020] If the first metal layer and the second metal layer of the storage capacitor C_(ST) corresponding to a pixel are short-circuited, the pixel capacitor is electrically disconnected from the data line such that no voltage can be applied to the upper electrode of the storage capacitor through the source or drain electrodes. Therefore, the capacitor voltage of the pixel capacitor C_(LC) is not equal to 0 but to a difference between the first common voltage Vcom1 and the second common voltage Vcom2, because the common voltages coupled to the pixel capacitor C_(LC) and the storage capacitor C_(ST) are different. By properly designing the voltage levels of the second common voltage Vcom2, it can be ensured that the difference between the first common voltage Vcom1 and the second common voltage Vcom2 is large enough to change the orientation of the liquid crystal molecules arranged between the front and rear plates of the pixel of the NW mode LCD panel. Accordingly, such faulty pixel will not always display its maximum brightness, and a bright spot is prevented from appearing on the liquid crystal panel.

[0021] In this embodiment, the first common voltage may be, for example, 4V and the second common voltage Vcom2 may be, for example, −5 volts. In this case, when the two electrodes of the storage capacitor C_(ST) are short-circuited and then the TFT (source electrode or drain electrode) is disconnected from the data line, the difference between the first common voltage Vcom1 and the second common voltage Vcom2 can cause the liquid crystal molecules to arrange in a direction parallel to that of the electric field. At this time, the liquid crystal molecules possess low light transmission rates, and the pixel does not form a bright spot on the liquid crystal panel. Instead, the pixel is totally dark, and the influence of manufacturing inaccuracy upon the display quality of the liquid crystal panel can be reduced.

[0022] In the pixel driving device for the LCD panel disclosed in the above-mentioned embodiment of the invention, the storage capacitor and the pixel capacitor may possess different capacitor voltage values, because they are coupled to common lines with different common voltage levels. Accordingly, when the first and second metal layers of the storage capacitor in a pixel are short-circuited and then the source/drain electrode is disconnected from the pixel capacitor, a capacitor voltage is maintained across the pixel capacitor. The level of the capacitor voltage is substantially large enough to arrange the liquid crystal molecules of the pixel in a direction parallel to the electric field, which are inserted between the front and rear plates of the NW mode TN LCD panel. Consequently, the brightness of such faulty pixel may be decreased, thus preventing a bright spot on the liquid crystal panel.

[0023] While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 

What is claimed is:
 1. A driving device for a pixel of a liquid crystal display, the driving device comprising: a scan line; a data line; a thin film transistor (TFT) including a gate electrode coupled to the scan line, a source electrode coupled to the data line, and a drain electrode; a pixel capacitor coupled to the drain electrode; and a storage capacitor coupled to the drain electrode and the pixel capacitor; wherein the pixel capacitor and the storage capacitor have different capacitor voltage values when a pixel voltage is applied to the pixel.
 2. The driving device according to claim 1, wherein the liquid crystal display is a normally white (NW) mode liquid crystal panel.
 3. The driving device according to claim 1, wherein the liquid crystal display is a twisted nematic (TN) mode liquid crystal panel.
 4. The driving device according to claim 1, wherein when an upper electrode of the storage capacitor and a lower electrode of the storage capacitor are short-circuited, a capacitor voltage of the pixel capacitor is maintained by disconnecting the pixel capacitor from the data line.
 5. The driving device according to claim 4, wherein a difference between the capacitor voltages of the pixel capacitor and the storage capacitor enables the pixel to display its minimum brightness.
 6. A pixel driving device for a pixel in a liquid crystal display, the pixel being coupled to a scan line and a data line, and the pixel driving device comprising: a thin film transistor (TFT) including a gate electrode coupled to the scan line, a source electrode coupled to the data line, and a drain electrode; a pixel capacitor coupled to the drain electrode; a storage capacitor coupled to the drain electrode; a first common line having a first common voltage level, wherein the pixel capacitor is coupled between the drain electrode and the first common line; and a second common line having a second common voltage level, wherein the storage capacitor is coupled between the drain electrode and the second common line, and when a pixel voltage is applied to the pixel, the pixel capacitor and the storage capacitor have different capacitor voltage values by being coupled to the first and second common lines respectively.
 7. The pixel driving device according to claim 6, wherein the liquid crystal display is a normally white (NW) mode liquid crystal panel.
 8. The pixel driving device according to claim 6, wherein the liquid crystal display is a twisted nematic (TN) mode liquid crystal panel.
 9. The pixel driving device according to claim 6, wherein when an upper electrode of the storage capacitor and a lower electrode of the storage capacitor are short-circuited, a capacitor voltage of the pixel capacitor is maintained by disconnecting the pixel capacitor from the data line.
 10. The pixel driving device according to claim 9, wherein a difference between the first common voltage and the second common voltage level enables the pixel to display its minimum brightness.
 11. A pixel driving device for a liquid crystal display, the pixel driving device comprising: a scan line; a data line; a thin film transistor (TFT) including a gate electrode coupled to the scan line, a source electrode coupled to the data line, and a drain electrode; a first power source; a second power source; a pixel capacitor coupled between the drain electrode and the first power source; and a storage capacitor coupled between the drain electrode and the second power source.
 12. The pixel driving device according to claim 11, wherein the first power source provides a first voltage level and the second power source provides a second voltage level, wherein the second voltage level is different from the first voltage level.
 13. The pixel driving device according to claim 12, wherein when an upper electrode of the storage capacitor and a lower electrode of the storage capacitor are short-circuited, a capacitor voltage of the pixel capacitor is maintained by disconnecting the pixel capacitor from the data line.
 14. The pixel driving device according to claim 13, wherein a difference between the first voltage level and the second voltage level enables a pixel associated with the thin film transistor to display its minimum brightness. 